Electromechanical band-pass filter

ABSTRACT

An electromechanical band-pass filter is provided with transducers to convert electrical energy to mechanical motion, or convert mechanical motion to electrical energy. Each transducer includes a magnetic member and a coil member movable relative to each other. The magnetic member is supported on a leaf spring for movement in a plane parallel to the surface of a ferromagnetic member. Each coil member comprises a flat coil mounted on a ferromagnetic member, each coil having straight conductor segments extending at a right angle to the relative motion.

United States Patent Hetzel Apr. 9, 1974 3,529,414 9/l970 Ganter et a]310/25 X Primary Examiner-Archie R. Borchelt [75] Inventor: Max Hetzel,Bienne, Switzerland Assistant Exammer-Marvm Nussbaum [73] Assignee:Elresor SA, Bienne, Switzerland Attorney, Agent, or FirmGriffin,Branigan and Butler [22] Filed: June 4, 1973 l N 366 590 [57] ABSTRACT[21 1 App An electromechanical band-pass filter is provided withtransducers to convert electrical energy to mechanical [30] ForeignApplication Priority Data motion, or convert mechanical motion toelectrical June 12, 1972 Switzerland 8684/72 gy- Each transducerincludes a g ic er June 12, 1972 Switzerland ..8685/72 and a oil membermovable relative to each other. 7 The magnetic member is supported on aleaf spring [52] U.S. Cl. 333/71, 333/30 R for movement in a planeparallel to the surface of a [51] Int. Cl. H03h 9/04, H03h 9/26ferromagneti member. Each coil member comprises [58] Field of Search333/71, 30 R; 310/25 a flat coil m u d n a ferromagnetic m h coil havingstraight conductor segments extending at a [56] References Cited rightangle to the relative motion.

UNITED STATES PATENTS 10 Claims, 6 Drawing Figures 3,485,035 l2/l969Ganter et al. 310/25 UX I s N 1? g N s 9 41 c 9 7 i I y Y '15 I I MEI 1BF 3 SHEET 2 BF 3 Fig.2

I 2 1: gig/7L w M N N RE: W

fATENTEDAPR 9 1974 SHEET 3 [IF 3 ELECTROMECHANICAL BAND-PASS FILTERthese resonators an electromagnetic transducer in form of a coil with apermanent magnet located therein is provided to convert an electricalsignal into a mechanical motion of the resonator.

The other resonator is also provided with a coil and a permanent magnetcore to convert mechanical motion of the second resonator into anelectrical output signal. As is usual on such electromechanical filters,the resonator is moved toward the coil and back therefrom, so that arelatively large air gap is required to prevent touching of theresonator at the coil. U.S. Pat. No. 3,480,809 shows a prior art filterof this type.

If the air gap between coil and resonator is large, the coupling factoris small. However, if a small coupling factor is present, the filtercannot be adjusted by means of electric terminating resistances to thedesired pass curve. As a result thereof only relatively small bandwidthsare possible because each resonator generates for itself a pass curve.If an attempt is made to increase the bandwidth, an undesired rippleoccurs in the middle of the pass region. A filter of relatively smallbandwidth or with a relative large ripple in the pass region is notsuitable for certain applications, for instance as the band-pass filterin a central remote control system, where low frequency signals aretransmitted over the power mains.

A further consideration is that the low coupling factor of thetransducer of the prior art filter has an effect such that the dampingcurve does not ascendto an infinite value at low frequencies, butattains a fixed value for the frequency zero, i.e., for DC current. Theprior art filter is therefore not capable of blocking low frequencies,which again makes this filter unsuitable for certain applications. I

It is an object of the present invention to provide an electromechanicalfilter which does not have the cited disadvantages and which,particularly in the lower low frequency region (100 to 1,000 hertz) hasa' relatively large bandwidth but only a very small ripple in the passregion of the damping curve.

According to the invention this is obtained on a filter of the typedescribed above by providing that each resonator comprises a leaf springto which a permanent magnet system forming the magnet member of thetransducer is attached, that the coil member of the transducer comprisesa flat coil lying in a plane arranged practically parallel to theoscillation plane of the resonator, that the permanent magnet systemcomprises a permanent magnet of high coercitive material located abovethe conductors of each half of the flat coil, both magnets beingconnected together by a yoke of ferromagnetic material and arranged insuch a way that the free ends of the permanent magnets are of oppositepolarities, and that the direction of magnetization of the permanentmagnets extends parallel to the plane of the leaf spring and verticallyto the plane of the coil, and that a ferromagnetic part is providedunderneath the flat coil to complete the magnetic circuit of thepermanent magnet system.

in a band-pass filter according to the invention, the air gap betweenpermanent magnet system and flat coil of each transducer can be keptvery small. The magnetic circuit of the transducer carries from onepermanent magnet over a short yoke to the other permanent magnet andfrom there over the air gap containing the windings of one half of theflat coil to the ferromagnetic part and from there again over the airgap with the windings of the other half of the coil back to the firstmentioned permanent magnet. This advantageous design of the magneticcircuit has only small losses and permits, in combination with permanentmagnets of high coercitive material, preferably a samarium cobalt alloy,the production of a very strong magnetic field in the air gaps. Thus, itis important that the permanent magnets magnetized vertically to theplane of the coil are not located adjacent to each other over a singlehalf of the coil, as is frequently the arrangement on tuning forkoscillators where magnetic coupling is of secondary importance. Sincethe permanent magnets are located at a distance from each other, thisprevents a large part of the magnetic flux from extending directly froma north pole to a south pole without vertically passing through thewindings.

The high electromagnetic coupling factor of the transducers of theinventive band-pass filter permits attainment of the desired relativelywide bandwidth. For a good filter it is important that it can beadjusted by electrical terminating resistances to the desired bandpasscurve to avoid the undesirable ripple in the pass region.

In prior art filters the use of high coercitive materials creates greatdifficulties or is impossible, because of the danger that the highattraction forces may cause the parts movable relative to each other totouch and block the resonator. On the other hand, in the band-passfilter according to this invention the direction of magnetizationparallel to the plane of the leaf spring has the advantage that theforces act edgewise on the leaf spring, so that it can hardly bedeformed by th high coercitive forces. In this way, a constantly, equalair gap is assured.

According to a preferred embodiment of the invention the ferromagneticpart carrying the flat coil extends on both sides further than the flatcoil in the direction of the relative movement of the magnet system andthe flat coil. The magnetic force lines of the permanent magnet systemcan enter practically vertically into the ferromagnetic part afterhaving crossed the relatively small air gap containing the flat coil,and by the relative motion between the permanent magnet on one side ofthe flat coil with the ferromagnetic part on the other side, themagnetic force lines are moved vertically to their direction back andforth. The magnetic force lines also retain their original configurationduring the movement because they are not deflected from their directionby the ferromagnetic part extending further than the coil. In this way,it is possible to obtain very high flux changes in the coil. Theferromagnetic part preferably comprises a material with small eddycurrent losses.

According to a preferred embodiment of the invention the windings of theflat coil comprise straight conductor sections located substantiallyparallel to each other and approximately at a right angle to therelative motion, the straight conductor section representing the activepart of the flat coil. This provides that in the operation of thetransducer the magnetic field remains substantially limited to thisactive part. In this way, a good coupling factor of the transducer isassured.

An embodiment of the invention will now be described by reference to thedrawing.

FIG. 1 is a top view of an electromechanical band filter comprising twoelectrodynamic transducers;

FIG. 2 shows a view of an electrodynamic transducer of anelectromechanical band-pass filter;

FIG. 3 is a sectional view taken along the line II of FIG. 2;

FIG. 4 is a perspective view of the basic parts of the electrodynamictransducer;

FIG. 5 shows a view of the transducer on which a leaf spring is used tomount the permanent magnet system at a small distance above the flatcoil; and,

FIG. 6 is a side view of the transducer shown in FIG. 5.

FIG. 1 shows a band-pass filter provided'with two electrodynamictransducers l and 1a. One transducer serves to convert electrical energyinto mechanical motion of a first resonator, and the other transducerserves to convert mechanical motion into electrical energy.

The mass of a permanent magnet system 2 serves together with the mass ofa leaf spring 41 as oscillating mass, so that the unit comprisingpermanent magnet system 2 and the leaf spring 41 represents a mechanicalresonator.

In the example of the band-pass filter shown, the leaf springs 41 areconnected to a support 43 which represents a mass coupling theresonators. Also mounted on the support 43 is part 5 (FIG. 3) with flatcoils 3. The whole is suspended on supporting means (not shown) bysuitable means such as thin rods 51, 53 and 55, for example. Othersuspension means may be used provided that a high coupling factor can beobtained.

The design and operation of a single electromagnetic transducer'of theband-pass filter will now be explained with reference to FIGS. 2 to 6.As can be seen from the drawing, the electrodynamic transducer basicallycomprises three parts, namely a permanent magnet system 2, a flat coil 3and a ferromagnetic part 5.

The flat coil 3 and the ferromagnetic part 5 form a mechanical unit. Thepermanent magnet system 2 is located above the flat coil 3 at a smalldistance in such a way that the permanent magnet system and the flatcoil can move relative to each other.

The flat coil 3 comprises a number of windings 7 which preferablycomprise insulated copper wire. The design of the coil as a flat coilhas the advantage that the air gap between the permanent magnet system 2and the ferromagnetic part 5 can be kept small. A flat coil can also besafely fastened in a simple way on the ferromagnetic part 5. Fasteningis preferably accomplished by glueing in such a way that the individualwindings 7 of the flat coil.3 extend parallel to the surface 11 of theferromagnetic part 5. The flat coil 3 has a form such that the windings7 have straight conductor sections 9, as best shown in FIG. 4. In theembodiment shown, each winding has two straight conductor sections 9lying in the flux region l3, 15 (FIG. 4) of the magnetic circuit 17(FIG. 3). These straight conductor sections form the active part of theflat coil 3. The straight conductor sections are located atapproximately a right angle to the relative motion.

The permanent magnet system 2 preferably comprises a plate-like part oryoke 21 on one side of which two arms are provided at a distance fromeach other. These arms, 23 and 25, are preferably permanent magnets. Thepermanent magnets may, for example, comprise a samarium cobalt alloy,and are magnetized in such a way that one magnet has at its free end amagnetic north pole and the other magnet has at its free end a magneticsouth pole. It would eventually also be possible to utilize a one piecepermanent magnet system, if a magnetic path 17 is provided which is ofsuch form that the magnetic flux lines extend approximatelyperpendicularly to the surface 11 of the ferromagnetic part.

The permanent magnet system 2 is designed such that the magnetic fieldis substantially limited to the active part, that is to the straightconductor sections 9, of the flat coil 3. This provides that themagnetic flux lines intersect, on a relative motion of the permanentmagnet system 2 and the flat coil 3, only the straight conductorsections 9 of the windings 7. This provides for a high coupling factor.

Either the permanent magnet system 2 or the flat coil 3 may form themovable part. It would also be possible to move both parts. However, itis necessary for the mutual influence that there by relative motionbetween permanent magnet system 2 and flat coil 3. This can be a backand forward movement. It is important that the air gap 29 remainsconstant during relative motion be tween permanent magnet system 2 andflat coil 3, and this is the case when the permanent magnet system 2 ismoved parallel to the surface 11 of the ferromagnetic part 5. To providea forward and backward movement parallel to the surface of theferromagnetic part 5 a leaf spring 41 (FIG. 5) can be provided, which isarranged in such a way that its flexing axis is perpendicular to thesurface 11 of the ferromagnetic part 5. Accordingly, the permanentmagnet system can only move parallel to the surface 11 of theferromagnetic part 5. According to the embodiment shown in FIGS. 5 and 6the ferromagnetiepart 5, together with the flat coil, is supported on asupport 43 comprising an arm 45. One end of the leaf spring 41 ismounted on the arm 45, whereas the other end of the leaf spring 41carries the permanent magnet system 2. In this arrangement of the leafspring the permanent magnet system can be moved parallel to the surface11 of the ferromagnetic part, whereas a movement in the direction of thecoil is impossible, because in this direction the leaf spring is stiff.Therefore, the permanent magnet system 2 cannot be drawn into contactwith the flat coil by the acting magnetic forces.

The electrodynamic transducer shown, particularly according to theembodiment of FIGS. 5 and 6, is admirably suitable for use in aresonator system of the type required in electromechanical filters.

For mechanical to electrical transducing, an induced voltage isgenerated at the terminals of the flat coil 3 according to the law ofinduction. This voltage U (FIG. 4) is proportional to the relativevelocity V, and the number of conductors N and the length Iof theconductors in the field.

Because the straight conductor sections 9 in both flux regions 13 and 15are connected in series it follows that:

One recognizes proportionality between velocity V and voltage U.

For electrical to mechanical transducing, a current I is applied to theflat coil 3 and a force K acts on the straight conductor sections 9 inthe flux regions 13 and 15 of the magnetic field lines B of this field.The force K is determined by the following formula:

whereby l is the length of the conductor in the field and N is thenumber of conductors. The force K accelerates the permanent magnetsystem 2 relative to the flat coil 3.

The described electrodynamic transducer is, because of its small sizeand its high ratio of magnetic field energy to magnetic mass,particularly well suited for band-pass filters requiring a relativelylarge bandwidth.

While a preferred embodiment of the invention has been disclosed inspecific detail, various modifications and substitutions in thepreferred embodiment may be made without departing from the spirit andscope of the invention as defined by the appended claims.

I claim:

1. An electromechanical band-pass filter having transducers forconverting electrical energy into mechanical motion of a resonator, andfor converting mechanical motion of a resonator into electrical energy,each transducer comprising a magnet member and a coil member movablerelative to each other, characterized in that:

each resonator comprises a leaf spring to which a permanent magnetsystem forming the magnet member of the transducer is attached;

the coil member of the transducer comprises a flat coil lying in a planeparallel to the oscillation plane of the resonator;

the permanent magnet system comprises a permanent magnet of highcoercitive material located above the conductors of each half of theflat coil;

both magnets are connected together by a yoke of ferromagnetic materialand arranged in such a way that the free ends of the permanentmagnetsare of opposite polarities;

the direction of magnetization of the permanent magnets extends parallelto the plane of the leaf spring and vertically to the plane of the coil;and,

that a ferromagnetic part is provided underneath of the flat coil tocomplete the magnetic circuit of the permanent magnet system.

2. An electromechanical band-pass filter as claimed in claim 1,characterized in that the permanent magnets consist of asamarium-cobalt-alloy.

3. An electromechanical band-pass filter as claimed in claim 1,characterized in that the windings of the flat coil comprise straightconductor section located substantially parallel to each other andapproximately at a right angle to the relative motion, said straightconductor sections representing the active part of the flat coil; andthat the permanent magnets are of such design that the magnetic fieldremains substantially limited to said active part of the fiat coil.

4. An electromechanical band-pass filter as claimed in claim 1,chracterized in that the ferromagnetic part extends on both sidesfurther than the flat coil in the direction of the relative movement, sothat the form of the flux path in the air gap between the permanentmagnet system and ferromagnetic part remains substantially the same inevery position of the relative movement thereof.

5. An electromechanical band-pass filter as claimed in claim 1,characterized in that the ferromagnetic part consists of a material withlow edge current losses.

6. An electromechanical band-pass filter as claimed in claim 5,characterized in that the ferromagnetic part consists of ferroxcube.

7. An electromechanical band-pass filter as claimed in claim 6,characterized in that the ferromagnetic part consists of lamellar iron.

8. An electromechanical band-pass filter as claimed in claim 2,characterized in that the windings of the flat coil comprise straightconductor sections located substantially parallel to each other andapproximately at a right angle to th relative motion, said straightconductor sections representing the active part of the flat coil; andthat the permanent magnets are of such design that the magnetic fieldremains substantially limited to said active part of the flat coil.

9. An electromechanical band-pass filter as claimed in claim 3,characterized in that the ferromagnetic part extends on both sidesfurther than the flat coil in the direction of the relative movement, sothat the form of the flux path in the air gap between the permanentmagnet system and ferromagnetic part remains substantially the same inevery position of the relative movement thereof. 1

10. An electromechanical band-pass filter as claimed in claim 9,characterized in that the ferromagnetic part extends on both sidesfurther than the flat coil in the direction of the relative movement, sothat the form of the flux path in the air gap between the permanentmagnet system and ferromagnetic part remains substantially the same inevery position of the relative movement thereof.

1. An electromechanical band-pass filter having transducers forconverting electrical energy into mechanical motion of a resOnator, andfor converting mechanical motion of a resonator into electrical energy,each transducer comprising a magnet member and a coil member movablerelative to each other, characterized in that: each resonator comprisesa leaf spring to which a permanent magnet system forming the magnetmember of the transducer is attached; the coil member of the transducercomprises a flat coil lying in a plane parallel to the oscillation planeof the resonator; the permanent magnet system comprises a permanentmagnet of high coercitive material located above the conductors of eachhalf of the flat coil; both magnets are connected together by a yoke offerromagnetic material and arranged in such a way that the free ends ofthe permanent magnets are of opposite polarities; the direction ofmagnetization of the permanent magnets extends parallel to the plane ofthe leaf spring and vertically to the plane of the coil; and, that aferromagnetic part is provided underneath of the flat coil to completethe magnetic circuit of the permanent magnet system.
 2. Anelectromechanical band-pass filter as claimed in claim 1, characterizedin that the permanent magnets consist of a samarium-cobalt-alloy.
 3. Anelectromechanical band-pass filter as claimed in claim 1, characterizedin that the windings of the flat coil comprise straight conductorsection located substantially parallel to each other and approximatelyat a right angle to the relative motion, said straight conductorsections representing the active part of the flat coil; and that thepermanent magnets are of such design that the magnetic field remainssubstantially limited to said active part of the flat coil.
 4. Anelectromechanical band-pass filter as claimed in claim 1, characterizedin that the ferromagnetic part extends on both sides further than theflat coil in the direction of the relative movement, so that the form ofthe flux path in the air gap between the permanent magnet system andferromagnetic part remains substantially the same in every position ofthe relative movement thereof.
 5. An electromechanical band-pass filteras claimed in claim 1, characterized in that the ferromagnetic partconsists of a material with low edge current losses.
 6. Anelectromechanical band-pass filter as claimed in claim 5, characterizedin that the ferromagnetic part consists of ferroxcube.
 7. Anelectromechanical band-pass filter as claimed in claim 6, characterizedin that the ferromagnetic part consists of lamellar iron.
 8. Anelectromechanical band-pass filter as claimed in claim 2, characterizedin that the windings of the flat coil comprise straight conductorsections located substantially parallel to each other and approximatelyat a right angle to th relative motion, said straight conductor sectionsrepresenting the active part of the flat coil; and that the permanentmagnets are of such design that the magnetic field remains substantiallylimited to said active part of the flat coil.
 9. An electromechanicalband-pass filter as claimed in claim 3, characterized in that theferromagnetic part extends on both sides further than the flat coil inthe direction of the relative movement, so that the form of the fluxpath in the air gap between the permanent magnet system andferromagnetic part remains substantially the same in every position ofthe relative movement thereof.
 10. An electromechanical band-pass filteras claimed in claim 9, characterized in that the ferromagnetic partextends on both sides further than the flat coil in the direction of therelative movement, so that the form of the flux path in the air gapbetween the permanent magnet system and ferromagnetic part remainssubstantially the same in every position of the relative movementthereof.